1. Field of the Invention
The present invention relates to a color proofing apparatus which utilizes an electronic signal input, and more particularly, to a method and apparatus for controlling the focusing of a writing beam in a thermal printer using lasers to prevent periodic focus excursions as a discontinuity in the media or the support is scanned past the focus detector.
2. Description of the Prior Art
Color-proofing is the procedure used by the printing industry for creating representative images that replicate the appearance of printed images without the cost and time required to actually set up a high-speed, high-volume printing press to print an example of the images intended. Ideally, these representative images, or proofs, are generated from the same color-separations used to produce the individual color printing plates used in printing presses so that variations in the resulting images can be minimized. Various color-proofing systems have been devised to create the proofs and have included the use of smaller, slower presses as well as means other than presses, such as photographic, electrophotographic, and non-photographic processes.
The proofs generated are judged for composition, screening, resolution, color, editing, and other visual content. The closer the proof replicates the final image produced on the printing press, as well as the consistency from image to image, from press to press, and from shop to shop, the better the acceptance of the proofing system by the printing industry. Other factors which influence the acceptability of proofing systems include speed, freedom from environmental problems, and cost of the system as well as the cost of the individual proofs. Further, since nearly all printing presses utilize the half-tone process for forming pictorial images, wherein the original image is screened, i.e. photographed through a screen to produce one or more single-color printing plates containing an image formed of a plurality of fine dots that simulate the varying density of the original image, proofing processes that employ the half-tone process to form an image are more acceptable to the printing industry than are continuous tone systems.
In recent years a variety of processes have been developed and implemented to electronically form, store, and manipulate images both for the actual printing as well as the proofing of images. While such electronic systems can handle and produce analog images, the most widely used systems employ digital processes because of the ease of manipulation of such digital images. In all of these electronic processes it is possible to display the resulting image on a CRT display, but it is still generally necessary to produce a "hard copy" (i.e. an image actually formed on a sheet of paper or other material) before that image can be assessed with confidence for approval of the final printing operation. Thus, all of these electronic systems require the use of some form of output device or printer which can produce a hard copy of the image for actual evaluation. It is to the field of proofing output devices that the present invention is directed.
While purely photographic processes can provide accurate reproductions of images, they do not always replicate the reproduction resulting from printing presses. Further, most photographic processes do not produce half-tone images that can be directly compared to the printed images they are supposed to replicate. Moreover, they are almost universally incapable of reproducing the images on the wide variety of paper or other material that can be run through a press. It is known that the appearance of the final printed image is affected by the characteristics of the paper or other material upon which it is printed; thus, the ability to form the proof image on the material actually to be used in the press can be a determining factor in the selection of the proofing system.
Other continuous tone proofing systems, such as thermal processes and ink-jet systems have been developed, but they have not been able to replicate the half-tone images so desired by the printing industry.
Electrophotographic proofing systems with half-tone capability have been introduced over the past few years which employ either wet or dry processes. The electrophotographic systems that use dry processes suffer from the lack of high resolution necessary for better quality proofing, particularly when the images are almost of continuous tone quality. This results from the fact that dry electrophotographic processes have not yet been developed which employ sufficiently small toner particle sizes that provide the requisite high image resolution. While wet electrophotographic processes have been developed that do employ toners with the requisite small particle size, these processes have other disadvantages such as the use of solvents that are environmentally undesirable.
In commonly assigned U.S. patent applications a thermal printer is disclosed which may be adapted for use as a direct digital color proofer with half-tone capabilities. This printer is arranged to form an image on a thermal print medium in which a donor element transfers a dye to a receiver element upon receipt of a sufficient amount of thermal energy. This printer includes a plurality of diode lasers which can be individually modulated to supply energy to selected areas of the medium in accordance with an information signal. The printer printhead includes one end of a fiber optic array having a plurality of optical fibers coupled to the diode lasers. The thermal print medium is supported on a rotatable drum, and the printhead with the fiber optic array is movable relative to the drum. The dye is transferred by sublimation to the receiver element as the radiation, transferred from the diode lasers to the donor element by the optical fibers, is converted to thermal energy in the donor element.
A direct digital color proofer utilizing a thermal printer such as that just described must be capable of consistently and accurately writing minipixels at a rate of 1800 dots per inch (dpi) and higher to generate half-tone proofs having a resolution of 150 lines per inch and above, as is necessary to adequately proof high quality graphic arts images such as those found in high quality magazines and advertisements. Moreover, it is necessary to hold each dot or minipixel to a density tolerance of better than 0.1 density unit from that prescribed in order to avoid visible differences between the original and the proof. This density control must be repeatable from image-to-image and from system-to-system. Moreover, this density control must also be maintained in each of the colors being employed in multiple passes through the proofer to generate a full color image.
Aspects of the apparatus which affect the density of the dots that make up the image include such things as variations and randomness of the intensity and frequency of the laser output, and variations in the output of the fiber optics which can vary from fiber to fiber and even within a single fiber as it is moved during the writing process. Variations in the finish of the drum surface as well as drum runout and drum bearing runout and variations in the parallelism of the translation of the printhead with respect to the axis of the drum will also affect the density of the image dots. The difference in the distance between the ends of individual fibers and the drum surface also affects image density because of the fact that the end of the fiber bundle is flat while the surface of the drum is curved. Temperature variations in the printhead due to the ambient temperature of the machine as well as the fact that the writing process itself heats the printhead also influence the image density.
Variations in the thermal print medium, or the writing element, such as variations in the thickness of the donor and receiver elements as well as the various layers that are a part thereof, can also affect the image density as it is being written.
Thus, it has been found necessary to continuously focus the writing beam as the image is being formed to assure that variations in the thickness of the donor and receiver elements, as well as other perturbations in the system, do not defocus the writing beam and adversely affect the image density or the sharpness of the image. Auto-focus systems have been developed utilizing the reflection of the writing beam, or of a separate focusing beam, from various surfaces of the writing element to focus the writing beam. Because of the nature of the writing beam, and the tolerances necessary to achieve the desired image quality, it has been found that the writing beam must be focused to within a few microns of a specific location in the writing element. Moreover, the limits of such focusing are further complicated by the fact that the lens used for such auto-focusing has a depth of field only in the order of microns.
In a commercial imaging apparatus the writing element is secured to a carriage, drum, or other support for the writing process. For ease of cutting and fitting the writing element to the drum, the writing element does not completely encompass the drum circumference, but leaves a portion of the drum uncovered between the leading and trailing ends of the writing element, which uncovered portion is perceived as a discontinuity by the auto-focus system. Ordinarily such a discontinuity would not create a problem since it lies outside the area in which the image is being formed. But where, as here, the focus of the writing beam is so critical and must be held to such close tolerances, and where the discontinuity is frequently scanned past the imaging beam, it has been found that the discontinuity can so adversely affect the auto-focus system that the focus cannot be immediately regained after the discontinuity has passed, so that the next portion of the image is out of focus for an indeterminate period of time. It is apparent that this is unacceptable, particularly when high quality proofing images are being formed.
Further, while a vacuum drum or platen provides a desirable means for mounting a writing element for this process, another means of initially securing the writing element to the support may be employed until the vacuum is applied. Such initial securing means may result in the formation of a discontinuity in the surface of the carriage or drum. Still further, in automated systems where both the donor element and the receiver element are automatically fed and secured to the support member, as well as released at the end of the writing process, may be advantageous to provide a "flat" in the drum surface to facilitate the automatic securing and releasing of the elements, which "flat" also forms a discontinuity.
Thus, it will be seen that a method and apparatus for preventing the auto-focus system from being confused by a discontinuity in the surface of the writing element or the support member is necessary to prevent the degradation of the image being written.